1. Field
The present invention generally relates to signal transmitters, and more particularly to a transmitter that employs multiple carrier modulation schemes (such as polar modulation and quadrature modulation) under different operational, environmental, or other conditions.
2. Background
The output power of code division multiple access (CDMA) wireless mobile transceivers must be tightly controlled over a significant dynamic range. Optimally, transmit power should rise and fall in harmony with the power of received signals. Namely when received signals are weaker, this might be because they originate from stations that are far away or because they are degraded by signal interference. In either case, this indicates a need to use greater levels of transmit power. Factors such as shadowing, fading, and simple transmission loss demand a wide dynamic range for a mobile station under power control.
There are many ways to modulate a transmitter's information onto a carrier. Quadrature modulation is a popular method. However, quadrature modulation tends to be noisy at high levels of output power, requiring substantial filtering to limit signal corruption. Nevertheless, with its economical power consumption, quadrature modulation is well suited to low output power regimes. Polar modulation is an alternative to quadrature modulation in which the amplitude and phase of the carrier are modulated directly. Polar modulation is better suited to high power levels than quadrature modulation, but performs poorly at low power.
Quadrature and polar modulation, then, have proven benefits under different circumstances. Conventional wireless mobile transceivers are designed to utilize the one modulation scheme that presents the most benefits and least drawbacks under the intended operating conditions. In fact, this conventional type of transceiver enjoys significant utility and widespread commercial use today.
Nonetheless, engineers at QUALCOMM INC. are continually seeking to improve the performance and efficiency of such mobile stations. In particular, QUALCOMM engineers have recognized that both polar and quadrature modulation schemes have different disadvantages, so that neither quadrature nor polar modulation is optimal for all dynamic conditions. As discussed above, though, wireless mobile transceivers are necessarily used over a significant range of transmit power levels, and these transmit power levels can change many times during a single call. Therefore, known wireless mobile transceivers are not completely adequate in this respect.